1. Field of the Invention
The present invention relates to a webbing retractor which stops rotation of a take-up shaft in a webbing pull-out direction, which rotation is caused by sudden pulling-out of a webbing.
2. Description of the Related Art
As locking mechanisms for webbing retractors which take-up, in the form of a roll, a webbing for restraining a vehicle occupant, there are a so-called WSIR (webbing sensitive inertia reel) and a VSIR (vehicle sensitive inertia reel). A WSIR senses sudden pulling-out of a webbing, and impedes rotation of a spool (take-up shaft) in the webbing pull-out direction. A VSIR senses a state of rapid deceleration of the vehicle, and impedes rotation of the spool (take-up shaft) in the webbing pull-out direction.
An example of the former mechanism will be briefly described. Both axial direction end portions of the spool, to which the proximal end portion of the webbing is anchored, are supported so as to be freely rotatable at side portions of a frame which is formed in a substantial U shape as seen in plan view. A gear, which is made of resin and is called a V gear, is mounted, so as to be relatively rotatable, to one axial direction end portion of the spool. An inertia plate formed of resin is assembled with the V gear so as to be relatively rotatable. A lock plate, which is formed in a substantial U shape as seen in plan view, is accommodated at the spool so as to be movable in the radial direction.
The lock plate as well is disposed within the shaft diameter of the spool since relative rotation usually does not arise among the spool, the V gear and the inertia plate. When the webbing is suddenly pulled out from this state, the spool, at which the proximal end of the webbing is anchored, attempts to rotate in the webbing pull-out direction. On the other hand, the inertia plate attempts or remain in its place. Thus, relative rotation arises between the inertia plate and the spool. A pawl, which is attached to the V gear, is thereby pushed by the inertia plate, and engages with internal teeth which are formed in a resin cover which is called a sensor cover. As a result, a state arises in which rotation of the V gear in the webbing pull-out direction is impeded, and relative rotation arises between the V gear and the spool. When relative rotation arises between the V gear and the spool, the lock plate is guided by a guide hole formed in the V gear, and the lock plate moves toward the radial direction outer side of the spool. Lock teeth formed at the distal end portions thereof engage with internal ratchets formed in the both side portions of the frame. In this way, rotation of the spool in the webbing pull-out direction is impeded. (Namely, a locked state arises.)
In the case of the above-described WSIR, when a vehicle occupant removes a tongue plate from a buckle device and the webbing is taken-up onto the spool by urging force, rotation of the spool in the webbing take-up direction is stopped at the point in time when the entire amount of the webbing is taken-up. Conversely, the inertia plate assembled to the V gear attempts to rotate as is in the webbing take-up direction due to its own inertial force. Thus, relative rotation arises between the inertia plate and the spool, and the pawl may engage with the internal teeth in the resin cover. As a result, there is the possibility that the lock plate will be guided by the guide hole of the V gear and the lock teeth of the lock plate will engage with the internal ratchets of the frame such that a locked state arises, and a state in which the webbing cannot be pulled-out thereafter (so-called end lock) will arise.